There is no perfect choice when it comes to air conditioning for a server room. The two primary determinants are the size of the server room and the thermal load of the equipment (the demand in BTU). Here are a few of the choices available for data center cooling:
Window air conditions
Although listed here, this is not really an option. The problem with window units is a lack of reliability and the inability for technicians to service the units easily. It might be the cheapest option, but it does not belong in the data center. It does not fit any of the requirements of industrial cooling.
Wall mounted split types
These are the most common types used for smaller server rooms or areas with the limited thermal load. These are meant for public spaces and not really designed for server rooms. As a result, they lack a humidifier.
Ceiling mounted air conditioners
These are designed for server rooms. However, they are limited to thermal capacity and can handle loads of up to 40Kw. Placement of these units is critical for servicing and to avoid any chance of liquid dripping onto equipment.
These units, used in conjunction with hot aisle containment are purpose-built for server room rack units. These are extremely efficient and do not require raised floors or any other special data center setup. They can even be purchased in ready made sets.